1. Field of the Invention
The present invention relates to a tension measuring apparatus for measuring the tension of a wire rod.
2. Description of Related Art
There is a demand for a technique for controlling the tension of a wire rod. For example, an automatic winding machine for winding a wire rod on a bobbin is required to maintain the tension of the wire rod constant for the purpose of increasing the quality and working efficiency.
Japanese Patent Application Laid-open No. 64S-41207 discloses a technique in which a tension control apparatus is used for controlling the tension applied to a wire rod by this apparatus so that the tension is maintained constant. More specifically, the tension control apparatus described in this patent document includes an arm for supporting a wire, a tension roller for applying a tension to the wire rod, a rotational resistance imparting mechanism for imparting a rotational resistance to the tension roller and a controller for issuing commands to the rotational resistance imparting mechanism. According to the technique disclosed in this patent document, the tension of a wire rod is generated by supporting the wire rod with the arm. Further, the tension of the wire rod can be changed at any time by changing the rotational resistance which the rotational resistance imparting mechanism applies to the tension roller based on the command received from the controller. The command issued from the controller is set taking into account the aging variation of the tension of the wire rod while the automatic winding machine is in operation.
That is, according to the technique disclosed in this patent document, the tension of a wire rod is controlled by supporting the wire rod with the arm, and adjusting the rotational resistance imparted to the tension roller by the rotational resistance imparting mechanism.
As explained above, the tension control apparatus described in the above patent document is configured to adjust the tension of a wire rod by controlling each of the support resistance of the arm and the rotational resistance of the tension roller. This conventional tension control apparatus has a problem that there may occur a control error due to deflection or vibration of the arm, or an error in controlling the tension roller by the rotational resistance imparting mechanism. Accordingly, this conventional tension control apparatus is not capable of controlling the tension of a wire rod with a sufficiently high degree of accuracy. Under the circumstances, there is a strong demand for a technique that enables accurately measuring tension of a wire rod.
It is known to use a load cell that converts a load into an electric signal for measuring the tension of a wire as the load. Such a load cell is installed such that its sensing part including a sensor is in direct contact with a wire rod so that the sensing part directly receives the tension of the wire rod. Accordingly, there is a concern that the sensor may be damaged by the tension of the wire rod.